Correlation of signal and reference data is important for many electronic products which require an ability to discriminate for coded signals. Examples of such products include global positioning system and geostar receivers, low probability of intercept receivers and other types of secure communications apparatus which rely on code discrimination as a means of signal recognition.
The performance of many such receivers suffers when lack of perfect correlation occurs, resulting in difficulty in synchronizing a reference code generator with a received code or in loss of received data. For example, if a code sequence is used comprising a plurality of bits, corruption of one or more bits is reasonably likely due to received signal fading, transient interference from natural or man-made sources, receiver noise and other causes. It has been especially uneconomical and impractical to digitally process imperfect correlation of a reference code with a received signal in order to allow system operation to proceed despite corruption of the received signal.
Thus, what is needed is a practical, economical method and apparatus for digital recognition of imperfect signal correlation, particularly a method and apparatus allowing system operation in the presence of real-world distortion effects.